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In the galaxy I'm worldbuilding for my science-fantasy series, by far the most important planet in the galaxy is an ecumenopolis called Ishga. What I had in mind for this planet is something resembling Coruscant from Star Wars, with either the entirety or the vast majority of the planet covered in a thick urban sprawl with many different levels and sublevels stacked on top of each other, some so far down that they are only lit artificially. However, Ishga differs from Coruscant in that a narrow band around the equator is left uninhabited, essentially a giant wildlife refuge where the original native wildlife of the planet can flourish. Even with this change though, the sheer amount of ultra-high-density urban land on the planet would give it a population in the trillions, which for the purposes of my book series, I cannot allow (Ishga's population far exceeding even two billion would put it at such an advantage compared to the other worlds that there would be no real stakes in the galactic war my series is about). How do I have an ecumenopolis like this but still have a reasonable compared to the other worlds (2 billion or less) population?

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    $\begingroup$ Are you asking for backstory justification for your vast, empty cityscape? Surely it would easier to just have something else cool but sensibly sized, instead of the aforementioned vast and empty cityscape? $\endgroup$ Mar 22 at 13:01
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    $\begingroup$ Build Ishga on a small moon, instead of a planet. $\endgroup$ Mar 22 at 13:49
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    $\begingroup$ I'm pleased you like my suggestions, but you should consider leaving questions open for longer before accepting an answer. You may get some amazing responses from people in other timezones! $\endgroup$
    – K. Morgan
    Mar 22 at 18:56
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    $\begingroup$ Downvoted because the question is nonsensical: if you have that sort of metropolis, or anything remotely close to it, your population is not reasonable. It's also not possible for it to be self-sufficient in food. (Or anything else.) $\endgroup$
    – jamesqf
    Mar 22 at 20:03
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    $\begingroup$ Trillions isn't even correct, it's more like quadrillions. An ecumenopolis on an Earth sized planet for 2 billion people is like building the Burj Khalifa to house a single person. Why did it become an ecumenopolis in the first place? I think you've missed the point of world building. $\endgroup$
    – Harabeck
    Mar 22 at 21:47

19 Answers 19

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Unchecked, the human population is likely to increase to the planet's ability to sustain it. So first let's look at reasons why it would be so small.

Option #1

A world of urban sprawl is presumably already mostly supplied by off-world colonies, so the limit would not be how much can be produced on Ishga but the supply logistics of bringing it in from off-world. There are only so many freight lanes to the garden colonies!

Option #2

Ishga is self-sufficient and has to produce all of its resources locally (i.e. no off-world supplies). If the planet is covered in buildings (and the nature preserve is well-policed) there's a limit to how much food and water will be available from rooftop gardens etc.

So the population is naturally limited, though some government control may also be necessary to avoid regular famine. Which leads to..

Option #3

The planet may have natural or artificial population control that means the birth rate does not exceed the death rate. This could be explained by the need to avoid civil wars, to avoid starvation, or all manner of social or religions reasons. Pick one!

Now you have to explain why the planet is covered in buildings

There are plenty of possible reasons for this too. Following Option #1, perhaps the entire planet is given over to ship manufacturing, or science labs, or exanet data centres, or whatever. Maybe half the continent is unpopulated, it's just some drone-filled Space Amazon warehouse.

Per Option #2, perhaps that land wasn't useful for food production anyway - previous generations turned it into a desert or a polluted wasteland, or perhaps it just was never fertile in the first place. So you might as well build big.

Per Option #3, maybe half the planet is covered in temples devoted to chasity?

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    $\begingroup$ Option #2 also has another consequence. Producing resources takes space. Think of warehouses full of hydroponics, giant power generators, water treatment plants, factories churning out everything from war machines to plastic toys, storage for the robots that maintain all of those and so on. If you combine this with a very hedonistic, wasteful society, you can probably fill all the lower layers with these and have the majority live in penthouses up top. Add the tendency of rich families having few children, you can have a planet planned for 100 billion with a population stagnating at 2. $\endgroup$
    – mlk
    Mar 23 at 9:07
  • $\begingroup$ "Unchecked, the human population is likely to increase to the planet's ability to sustain it" -- In a population with high living standards and education the birth rate isn't necessarily going to be above the replacement level, see the demographic transition in real life. $\endgroup$
    – Hypnosifl
    Mar 23 at 16:31
  • $\begingroup$ The Earth could easily support a trillion people or more. The solar system alone can easily support quadrillions, quintillions if we allow for digital existence. The only limit is heat dissipation, and with proper engineering that can be "solved" as well. $\endgroup$ Mar 24 at 13:04
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Getting to the root of the problem:

I am trying to follow why Ishga having a large population is a problem. You have already established that it is the most important planet for some reason. If it is the hub of manufacturing, it is disproportionate in a war. If they control trade, they are disproportionate in a war. If they have an invaluable strategic position, they are disproportionate in a war. People have nothing to do with it.

Why wouldn't a large population be a disadvantage? Your city could be overburdened with intelligent life, and the stresses of it all make large portions of your world-city a burden. So rather than being at an advantage, Ishga is consumed with its own problems despite once being the great galactic police force to stabilize everything. Like a Great Britain completely dependent on its supply lines, Ishga has to constantly use all their strength to keep things open and unchanging. A single day of supply disruption means famine and civil war.

The only situations where their population is an advantage are if:

  1. voting by intelligent life forms influences the outcome of the wars (unlikely, but not impossible; have political parties divide the planet's votes to make them impotent), or
  2. war is somehow dependent on personal abilities, like psychic powers, or the technology used in war is restricted by legal means and therefor population represents power (and again, like foreign fighters getting involved in Afghanistan, the Ishgans are so diverse that they divide their loyalties).

The most likely solution to your problem would be that Ishga is a giant mass of factories and warehouses where the stored wealth of the galaxy sits amongst the palatial mansions of trillionaires (likely overlooking the nature preserve). Machines and robots do all the useful work, and thus you don't need a significant number of people on your planet. The problem with this is that realistically, this would make Ishga the overwhelming power in the galaxy in any war. Vast hordes of automated tanks, planes, fighters, and battleships pour forth from their massive factories. They dominate everything. Again, the number of people is small, but people don't represent power in the future.

Your best hope is that such an Ishga has divided government. Each trillionaire is an island to his/her/its own family. Every planet is represented here, and any government is designed to maintain the status quo without interfering with the power and freedom of the citizens. Thus Ishga isn't really an independent planet, but more of a safe haven for wealth and the unrestricted source of all the weapons fueling the war. The planet sells weapons to both sides of every conflict and is really a non-issue in settling conflicts.

You can certainly ADD a huge, seething mass of people to the planet if you want. The powers that be keep a vast mass of beings alive on the dole, sitting around and fighting amongst themselves or consumed with artistic (i.e. non-productive) pursuits. But there is no need for them. Make the population anything you want. They don't really matter.

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Tiny Planet

So, if you want a mega-city where the light never reaches the lowest levels, you need a population density that at least meets the highest population density of any city on Earth. Right now, that's Monacco with 26,000 people per sq km.

If you need the population below 2 billion, then let's make a high estimate of 75,000 sq km of megacity (smaller than the US state of South Carolina). You want a band in the middle for wildlife, so let's assume that the Megacity only actually covers 50% of the surface of the planet.

This leaves your planet with a surface area of 150,000 sq km, so it would need a radius of about 110 km.

If the entire planet were made out of iron, the gravity on that planet would be roughly 1/50 the gravity of Earth.

Frame Challenge: What About a Habitat?

Instead of a planet, what if you instead made it an artificial habitat orbiting an otherwise uninhabitable planet or gas giant?

A population of that size and density would fairly easily fit into a McKendree cylinder which is just a scaled up version of an O'Neil cylinder.

These sorts of cylinders create their own gravity by spinning and have to exist in pairs in order to maintain a steady orbit.

Considering the hypothetical upper limit on size for a McKendree cylinder has 13 million km2, you could make a much smaller pair with more than enough room for 2 billion people and the agricultural area to support "wildlife."

Depending on how old your stellar landscape is, you could even add a bit of history about the cylinders being built to escape some sort of planet-wide calamity (perhaps the result of an old war). That would allow you to make the artificial biomes in the cylinders unique to Ishga and representative of the species that used to be native to their planet.

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    $\begingroup$ You seem to have a math error somewhere: for a surface area of 150,000 sq km, I get a radius of 110 km. $\endgroup$
    – Mark
    Mar 22 at 20:40
  • $\begingroup$ Oh whoops, so I do. I think I fixed the rest of it too then. $\endgroup$
    – SirTain
    Mar 23 at 12:45
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    $\begingroup$ And then you can also make it a fully armed and operational battle station! $\endgroup$ Mar 23 at 14:47
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The population is enormous. It's also uncontrollable.

One solution to this problem is to say that the population really is enormous, but that with so many people packed into such a large area, it becomes unmanageable by the government to control that many. Most of the planet is controlled by various gangs with no interest in helping the war effort; instead, they spend their time and effort fighting each other, and if the government were to try to coerce them into helping, they might stop fighting each other and start fighting the government - a fight they would win, since they heavily outnumber available government forces.

Of course, that also means that the planet would be just an uncontrollable by any polity that were to invade the planet - but if the government's forces get overthrown and replaced by a new government ruled by different people, the gangs wouldn't care as long as they were left alone to continue their business.

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    $\begingroup$ I like this. Technically it's a planet of trillions of people, but they're only counting the people that actually matter. People who contribute to the world's GDP in some way. If most of the planet is effectively anarchic gangs and slums and only the literal upper-crust living in their (less literal presumably) ivory towers actually participate in civilisation, then the planet only has a relevant population of 2bn or so. $\endgroup$
    – Ruadhan
    Mar 23 at 8:08
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    $\begingroup$ The factions don't have to be gangs. All sorts of large organizations -- noble houses, mega-corporations, local governments, and representatives of other planets -- can have their own enclaves, and their interests will not always align with each other. $\endgroup$ Mar 23 at 18:47
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The last war has left the planet a Mass Grave

A long time back, the galactic landscape was significantly more populous. Your ecumenopolis thrived with a teeming population of trillions.

That is, until the last war.

With supply-lines cut, the people starved, the planet was completely unable to sustain its population and most of them died.

Entire areas of the city went dark. Without anywhere to bury the bodies (or people to safely handle them) buildings were sealed up with millions of corpses in them.
Diseases ran rampant due to the profusion of dead bodies, killing billions more.

Most of the planet is now a mass grave.

The survivors have recovered for the most part, supply-lines are back, the city now grows a not-insubstantial amount of its food in bio-domes and there is a quiet attitude of "never again".

Sometimes a building is unsealed and cleaned out for habitation, but nobody wants to disturb the dead more than necessary.

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  • $\begingroup$ I also had this thought. That being said, this can be modified to something a bit less "dark". Ishga might've been the capital of a solar/galactic empire in ages past but then "something" happened (what that "something" was might even have been forgotten by now) and the empire crumbled. With it, Ishga lost its supply lines and started suffering from frequent famine and civil unrest. However, this didn't necessarily lead to the planet becoming a mass grave, but simply that the vast majority of the population left for greener pastures in the former garden colonies. $\endgroup$
    – Wasabi
    Mar 24 at 13:47
  • $\begingroup$ I'd love to see the space-lift infrastructure that could transport trillions of people off a planet faster than they can replenish by natural birth-rate! But yes. A concerted effort to do that while the supply-lines diminished over time would be an alternative. $\endgroup$
    – Ruadhan
    Mar 24 at 13:58
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Build Ishga on a small moon, instead of a planet

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Have the city built by other

The occupants of the city are not the builders of the city. The city is and was being constructed by many autonomous builders. The planners and architects are long dead and gone(10K years+). No finish state was defined/reached for the builders so they keep building. They are old and mysterious enough nobody knows how to tell them to stop. The builders would be an outsized faction if they were not so single minded focused on construction and maintenance of the city.

This allows for a much smaller population then the city suggests and with the population not controlling the builders they don't have outsized economic power. Which would mean an organization possibly gaining control of builders would be a plot point.

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It sounds like you are following the path of Solaria in Asimov's The Naked Sun.

I think you can achieve this in a similar way as in that series:

  • strict control on the reproductive behavior through education, to the point where intercourse is seen as a mere formality to be obliged as a social duty and physical interaction in general is frowned upon
  • large availability of robots for taking care of all the tasks, so that everybody can enjoy their isolated life.
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    $\begingroup$ This doesn’t really solve the main problem. There are lots of ways to keep the population down, but the questioner wants an enormously large built-up area despite a low population, and that’s the hard thing to explain. Solaria had scattered estates with large swaths of countryside both within and between them. $\endgroup$
    – Mike Scott
    Mar 22 at 11:10
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    $\begingroup$ @MikeScott: It does solve it if the robotic servitors require a large industrial base to keep themselves and the city they inhabit running. Then you end up with isolated humans living in pockets of luxury inside the vast, sprawling factory that is required to maintain their lifestyles. $\endgroup$
    – Joe Bloggs
    Mar 22 at 13:28
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Necropolis

Perhaps your planet is "most important" by being the spiritual center of the galaxy as well as whatever else it has going for it. Beings long to be buried there, and for whatever cultural reasons the majority prefer mausoleums to simple graves or being cremated or being crushed down to a synthetic diamond or what have you. At the extremes, you could have people buried in what amounts to exact replicas of towns or mansions or what have you. Or heck, you could even go crazy to the point of "the being's body was crushed into a synthetic diamond, placed in the forehead of a statue of said being, and being was placed in the "Human" sector of Ishga's Necropolis 7, which is an exact replica of Tokyo in 1995." Except that it sits a kilometer above the actual ground-surface of the planet, and 3 kilometers below where you can actually see the sky. Something like this would be the only way I can think of to cover a planet in a "city" yet keep the population at a somewhat steady 2 billion.

The 2 billion population would be mostly those involved with maintaining/constructing mausoleums and people administering the planet/empire. If you want pilgrims that could give you a way to get added billions (or evacuate billions, or whatever). If you don't want pilgrims a simple "you must be the Chosen Caste or a corpse to set foot here" would solve most of that problem.

Of course the OTHER way of getting a world-spanning planet with a paltry 2 billion population is to have some nanite "virus" or other artificial plague which took the planet down from the hundreds of billions in the past. But that seems like it'd lead to even more problems than my first idea.

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  • $\begingroup$ Welcome to Worldbuilding! $\endgroup$
    – JDługosz
    Mar 22 at 15:02
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A urban area is defined as an area with a population density of 193 people/km^2. If we go with the max population size of 2 billion people, as well as the minimum population density, then the planet's surface area would be $1.04*10^7 km^2$. For comparison, Earth's surface area is $5.10*10^8 km^2$, so that means that the planet's surface area is 2.03% of Earth's, and its radius is .143 Earth radii, so it would be a rather small world. Ishga would probably need an artificial magnetosphere in order to keep an breathable atmosphere.

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It may not be possible. A lot of researchers have suggested that the human population right now is overshooting carrying capacity by quite a bitv(actual sustainable population over the long term may be anywhere from two to six billion), and ecumenopolises are heavily dependent on where they get their food from. You would have to have a significant amount of space dedicated to food and energy production if nothing else.

By energy, I mean things like solar power, because a mass burning of fossil fuels would be both economically and environmentally unreasonable (you'd have to be burning several Earths worth of coal and oil to keep the planet heated, which would have to be imported from somewhere, and just think of the global warming and pollution from that)

By food, it's been pointed out by many people that you would have to have local means of food production. Importing food from offworld would be hugely expensive and a logistical nightmare and would also make your capital planet incredibly vulnerable to attack in the event of a blockade. Society is three meals from collapse and all that. At the first major sign of war your equatorial wildlife refuge would be eaten just like how starving Parisians ate the Paris zoo in 1870 (or like how people in Venezuela are eating zoo animals now).

Just for reference as well, two billion people is about the size of the human population pre-Industrial revolution, when there were very few large cities and most people lived on farms.

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The planet's main activity is some room-intensive (but not labour intensive) activity

I agree with the idea that you could ramp up the population of the planet without making it a powerhouse: too a high population is a liability under some circustance, since they must be fed and also the task of managing such a big infrastructure would be very challenging.

Also, 2 bilions people would require a lot less room than an entire planet: even if I am very rich, I wouldn't need an entire block of skyscrapers just for me, it would be boring! Ultra wealthy people usually live in big estates, but usually the greatest amount of space is occupied by open-air structures, such as pools, sport fields, parks and gardens...

So, the idea is that you need some economic activity that needs a lot of enclosed room, but very little manpower. A planet completely designated to that activity would be covered by a lot of such facilities (both for the finished produt and its supply chain), while requiring a "modest" population.
For instance, they could build warp engines: These engines are huge and require some thousand square meters-sized clean room in order to build them (and also their components require a lot of space), while, thanks to the automatized manufacturing, only a handful of people are needed to manage the factories. There are hundreds of thousands of such factories on the planet, because for some reason it is the best place to build the engines (maybe because there is abundance of some unobtanium needed for them), and the demand for warp engines in a galaxy can be huge!
This way the planet is almost completely covered in structures, even if the population is just a pair of bilions people (the people needed to operate the factories, plus all the people who must manage the society): the population can't go beyond some threshold, because there would be just a limited amount of living space on the whole planet, since the factories have the priority.

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Plateau syndrome.

Your planet's geology is a little different, and water is scarce. The plate tectonics create ocean trenches filled with sea water, but only the "sea bed" has enough air to be habitable. The "continents" are a tall plateau with only a few kPa of air. The "sea bed" is actually only a few percent of the planet, some of which has widely been set aside so that there is still some air to breathe next year. It has been very extensively developed, and due to this vertical landscape and concentration of humans in a small space (not to mention a lack of other employment), it became well suited for a centralized bureaucracy.

Duels.

There are over ten thousand slurs, epithets, taunts, fighting words, and politically incorrect abuses of language on the planet, each of which can be expatiated only by a Duel, fought to the death naturally. (What other way is there to say you're sorry??) The civilization used to be much more cosmopolitan and developed, but now the populace hides among the well-built towers of yesteryear ... trying to avoid notice.

Cats.

Your planet is the Capital of the Galaxy. The richest people from all the cosmos live there. And they build tall, tall climate-controlled towers staffed with the finest robots and security drones to tend to all of their beloved furry companions. Still, a person with only one mile-high skyscraper full of cats (and rookeries of birds for their entertainment) does not feel very distinguished. That sort of person isn't keeping up with the Joneses and his next promotion at the Bureau of Galactic Charity Regulation may be in doubt.

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The buildings were built by robots that didn't care about population.

If some terraforming process also made the buildings (and not the populous), it may have either miscalculated or not cared about the current population.

This leaves you with either a rogue AI terraforming up buildings, or a disinterested population who are happy to live either side of the equatorial wild life refuge and not explore.

Perhaps the disused buildings are non-sensicle in their construction, with buildings unfit for purpose. Or they've fallen so far into disrepair that no one wants to expand into the tropical wastes.

It seems so rational to assume that the population would slowly spread out to fill the available space. But thinking-beings are anything but rational.

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Maybe property prices are just too high so few can afford to live there, and those that can afford to live there have a lot of stuff. A LOT OF STUFF. All dedicated to their own personal use. And all that stuff takes space and produces waste heat and at some point, you can't have any more waste heat because you can't remove it from the planet fast enough.

What kind of stuff might a single person have that might take so much space and produce so much heat? Well, what if each ultra-wealthy person had several dozen power plants all dedicated for their own personal use? And farms and food processing facilities all dedicated to them and only them? And water filtration and sewage processing? And of course, refineries and factories to produce whatever goods they wanted from toiletries to furniture to computers to vehicles to starships? They would also need very own dedicated infrastructure to support such facilities and move around such facilities. And a private lake, of course. Or better, yet island? And of course, they would need their own personal space port and mass driver get my stuff in and out of orbit, because why would they share with anyone else? Sharing is for peasants. If all 2 billion people had these? That's a lot of space, and a lot of waste heat.

The Earth has a surface area 510 million square kilometers. With 2 billion people, that leaves only a paltry 255 square kilometers per person use per level of the ecumenopolis. That's a 16km x 16km square. It doesn't even cover a quarter the area of a city. You couldn't fit all that stuff in there with just a single level for all 2 billion people. I think you would need a few levels at least, giving you a reason to have an ecumenopolis. The space port alone would probably take up an entire 16x16km layer, let alone all the stuff to support it.

And of course, you can always use some aristocratic mumbo jumbo where they purposefully keep people off for no good reason on than their own whims.

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Demographics and/or economics

Make the planet rich

It's a problem that has already been solved by itself. That's because countries with the highest human development index (HDI) are the very countries with sub-replacement fertility rates.

In other words, well developed societies have a diminishing population. You will have to import new people continually or otherwise your planet will crumble.

Make another thing more scarce

If this is not very convincing, simply make the food really expensive. And it's easy in an ecumenopolis.

Previous urban land was developed in the past, when food was cheap. All food needs to be imported then and now, where all is either de the city sprawl or raw wildlife.

Food got expensive over the years, so now very few rich people can live comfortably here.

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Most of the buildings are data centers and automated fabrication facilities.

Perhaps the further you are from the green band the lower the population gets.

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The fundamental problem is that if you want the feel of a multi layer city with huge population density, you need ... huge population density.

If you want a population of under 2 billion, and the density is constant, this means the area must be small.

If the area is small, and the area needs to cover the entire surface of the Ecumenopolis.

Suppose you want 10 layers that are each as densely populated as Manhatten is: 25,000 people per square km.

$\frac{2 billion}{10 * 25,000 / km^2}$ is 8000 $km^2$.

The formula for the surface area of a sphere is $4 \pi r^2$, giving us:

$$8000 km^2 = 4 \pi r^2$$

giving us a radius of 25 km. This is 250x times smaller than Earth.

This means its density has to be 16 times greater than Earth's, or 88 tonnes per cubic meter.

The densest materials cap out in the 20s of specific gravity.

You say you want a strip over the equator. We'll say that 3/4 of the surface is not city, either the strip, or open water, or something else. That raises your radius up to 50 km, so only 125x larger than Earth, and the density drops to 63.

Regardless, the material this planet like thing has to be made out of is going to have to be extremely exotic. You can't even say "they built it around a fragment of neutron star", because all of the high density stellar remnants hold themselves together with their own gravitational pressure, which requires a surface gravity way higher than 1 gravity. Small amounts of exotic stellar material just explode and boil off, they aren't stable.

On top of that, building a planet around a mote of white dwarf or neutron star exotic material doesn't work very well. Any kind of suspension will be unstable, and containment of that exotic matter is crazy.

So instead, what if the city was built on an artifact of unknown origin. It was wrapped in raw materials (metals, etc) to a crazy depth. The original settlement, ages ago, mined those metals; over time, settlements arose. Eventually they dug down to the exotic core (which is near the size of the micro planet) and have exploited its strange properties to gain economic advantage. The copious resources on the planetoid and the unique technology allowed by using the exotic core as a catalyst led to a growing amount of urbanization, and eventually swallowing up much of the planet in city.

While the surface gravity is near Earth levels, the shallow gravity well makes entering and exiting the gravity well much cheaper. Air pressure remains a serious problem, with huge reactors converting massive amounts of mined material into gasses covering much of the surface.

Now, suppose we want a non-exotic material density. Iron has specific gravity of 8; or 11x less dense than the 25 km model.

11x less dense means 11x larger radius to get the same surface gravity (the math works out, really), so we need a radius of 275 km. This reduces population density by a factor of 121x, going from 10x Manhattan to 0.1x Manhattan.

If there is a band around the equator that takes up 75% of the surface area of the planet, and half of the poles are ocean, we get back up to 0.8x Manhattan population density.

If 95% of the industrialized surface is factory areas with a density of 50% of the City of Chicago, the remaining 10% has a density of 7x Manhattan.

This "only" results in a population density of 1 per 6 m^2. If we have 20 layers on average and 50% transportation overhead (typical in real cities), each person has 240 m^2 to live and work in, or 2600 square feet.

20 layers on average in the "urban" parts of the planet isn't bad, and 2600 square feet to live and work is reasonably cramped.

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  • $\begingroup$ If you relax your population density from that of Manhattan to that of an area with heavy industry, the required density will approach that of iron. Perhaps the planet is the core of a planet like Earth, with the outer molten rock layer stripped away in some ancient collision. This is the perfect planet on which to set up heavy industry - mining is easy, because the whole planet is made of iron, (with a fair proportion of tungsten, osmium and other very heavy elements in it.) Probably some area is given over to mining as well. $\endgroup$ Mar 25 at 1:55
  • $\begingroup$ @LevelRiverSt Did the math on that. With a thick band around the equator, a fair amount of ocean at the poles, 95% of the surface being industrial (but with an insane population density for industrial; both workers and irregular settlements), the remaining 5% can be somewhat cramped with 20 layers of urban population. $\endgroup$
    – Yakk
    Mar 25 at 3:18
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Manmade Ecological Disaster / Geomagnetic Reversal

My immediate mental image upon seeing the question was Chernobyl, where a technological incident left a previously inhabitable area uninhabitable. Especially if it was fast, but not sudden (say a week or so warning), there could have been a mass exodus from the planet, with the remaining people being far away from the disaster.

A related concern is Geomagnetic Reversal, where a planet's magnetic field flips, temporarily damaging it near the poles, bringing dangerous solar radiation to non-equatorial regions.

While either could explain the exodus of people from the citadel world they built, you might consider combining the two. Geomagnetic Reversal takes hundreds of years, so perhaps they developed a massive device to bolster the magnetic field during the transition. If such a device were damaged, there would need to be a sudden evacuation of the polar regions before radiation became lethal.

This would explain why buildings were left intact, why wildlife could flourish in the equator, and why a planet with infrastructure for the trillions would have so few people.

Perhaps the last survivors are the uber-wealthy, who bought up the safe land in what's practically an archaeological site. Alternatively, perhaps only the poor live there, unable to afford a safe planet and forced to live in an irradiated tomb world? Maybe it's archaeologists/treasure hunters, ecologists who study and maintain the surviving wildlife, or mega-corporations whose use of automation means they can safely use the planet for construction. Maybe politicians live in a galactic capital city that should have been moved off-world years ago. There's lots of room for different reasons why someone would live in the shadow of such a massive world, and it leaves you open to tell your own story.

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